Tuesday, October 7, 2014

Techy Tuesday - New Fiber Optic Speed Record

It will take a while to make it down to the commercial sector, but a new world record for speed in a fiber optic link has been set. The High-Speed Optical Communications (HSOC) team at Denmark Technical University (DTU) has announced a new fiber-optic technology that transmits at 43 Terabits per second (Tbits/s) using a single laser source.  The team broke the existing 26-Tbit/s record set by researchers at Germany's Karlsruhe Institut für Technologie.

The details are a bit esoteric and I need to talk a little about how data is sent over fiber optics.  To begin with, most fiber optic lines are capable of carrying many different colors of light.  They are optically big compared to the wavelength of light, so light bounces around in there without regard to color (and optical guys always refer to wavelength, not color).  It's literally like wire; you can put many frequencies on it and they propagate down the fiber just as easily.  Electromagnetic field guys would say that the light's EM field can take on many different patterns, making it a multi-mode fiber.  In contrast, there are also single mode fibers which are more selective in what they transmit, only allowing certain frequencies.  It's intended for single wavelengths and one mode of transmission, but other wavelengths can fit in there, too.  Usually shorter wavelengths/higher frequencies; in light a green wave can fit in a red single mode fiber, or blue in green single mode fiber. 

Radio guys will note the analogy to coax and waveguide.  Multimode fibers are more like coax while single mode fibers behave more like waveguide.  The data transmission in multi-mode fiber tends toward the simpler types: on off keying; simple codes.  The data sent in single mode fiber is often more like other data communications at radio frequencies. 

What's novel about this is not just the speed, but the approach.  The DTU team used an optical fiber with seven single mode fibers in it, an idea from NTT.  One of the fiber paths was used to synchronize the other six, and each of the six carrying a 320 Giga-baud (billion baud) channels on a different color light (called Wavelength Domain Multiplexing).  The six WDM channels were then polarization-multiplexed (that is, one set of data are polarized in one direction, while the other set of data takes an opposite polarization) with each optical pulse carrying two bits of information in quadrature using the differential quad phase shift keying (DQPSK) format.

(generic artsy picture of optical fibers that don't have anything to do with this news - source

How much data is 43 Terabytes?  In the last ten years or so, someone coined the phrase Library Of Congress for large amount of data.  One LOC is 10 Terabytes or 10,000 gigabytes.  This system would transmit 4.3 LOCs in one second. At 50 GB in a Blu-Ray DVD, 43 Terabytes is 860 Blu-Ray DVDs per second.  Look at it this way, that 5 Zettabyte data center the NSA just built in Utah?  It would take 116,279,070 seconds to fill it with this data hose.  1346 days.

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